But
they are not necessarily favored to have the race “called” for them on Tuesday
night. Because of a variety
of circumstances like possible
runoffs in Georgia and Louisiana and the potential decision of
Kansas independent Greg Orman about which party he chooses to caucus with, the
outcome of the Senate may not be determined until days or weeks from now. The
forecast refers only to the probability that Republicans will eventually claim
control of the Senate by the time it convenes in January.

He adds that:

… the
polls have moved toward Republicans in the closing days of the campaign —
making their position more robust. The movement has been clearest in states
like Kentucky, Arkansas and Georgia that typically vote Republican, suggesting
the election may be converging toward the “fundamentals” of each state.

If you prefer it in visual form, here's Silver's graph:

For further discussion, see Silver's extensive analysis of the data in his post.